function [J B Ae]=CST_B(xy)
	NN=[1 0 -1
		0 1 -1];
	J = NN*xy;
	Ae = 0.5*det(J);
	A = 1/det(J)*[J(2,2) 	-J(1,2) 	0 		0
					0 		   0 	 -J(2,1)  J(1,1)
				-J(2,1) 	J(1,1) 	  J(2,2) -J(1,2)];
	G = [1 0 0 0 -1 0
		 0 0 1 0 -1 0
		 0 1 0 0 0 -1
		 0 0 0 1 0 -1];
	B = A*G;

end